Cool Roofs Protect the Environment and Save Money
Energy Secretary Dr. Steven Chu recently made headlines by calling for the widespread use of "cool roofs" as a smart way to combat climate change.
The idea was oversimplified in the news media as simply "painting the world white," but that is not what Secretary Chu suggested. In fact, it is a caricature of what could be an important way to offset our carbon emissions. Secretary Chu is correct in suggesting we pursue cool roofs, and I hope more people will learn about this new strategy and consider adopting it for their homes and businesses.
If nothing else, a white or cool roof will save you up to 20 percent on your air conditioning bill and it's hard to argue with that. Over its lifecycle, a new white roof costs no more than a traditional roof.
The basic idea behind cool roofs is simple and recognized for centuries by the ancient Greeks and Egyptians. Dark colors absorb more heat than light colors. It's for this reason that people living in the tropics wear light-colored clothes and the same reason you don't lean on a black car on a hot day. Similarly, darker colored roofs retain more of the heat from sunlight within our atmosphere. But light roofs reflect more of that light straight back into space. Therefore, making roofs lighter in color increases their solar reflectivity and directly offsets CO2 emissions.
The potential savings are both huge and surprising. My colleagues and I have estimated that replacing urban roofs with solar-reflective materials in tropical and temperate regions of the world would offset 24 billion tons of CO2.
Let me explain. The average US roof is approximately 1,000 square feet and lasts for about 20 years. A white roof produces a one-time offset of 10 tons of CO2 and would eliminate emissions from one car for more than 2.5 years. Considered on a national scale, the equivalent would be eliminating two billion tons of CO2 emissions or removing 20 million cars off the road for 20 years. From a global perspective, replacing dark roofs with cool ones would be equivalent to taking half the world's cars - 300 million vehicles -- off the road for 20 years or reducing 24 billion tons of CO2 emissions for the same period.
That may sound too good to be true, but it is possible.
Because most large, modern cities have dark roofs, roads, and parking lots, they tend to run 5-10% hotter and create the "urban heat island" effect. Cool roofs mitigate the "urban heat island" effect and improve outdoor air quality and comfort. Light-colored roofs have other benefits. Most importantly, they lower temperatures inside of homes and businesses, thereby reducing the need for air-conditioning during the hot summer months. That translates to additional savings in CO2 emissions.
Since 2005, California building standards have required that any flat roof on a new building be a white roof. The California will soon also require new residential roofs to have cool colors as a way to reduce cooling costs. In an effort to cut its power costs, the city of Phoenix recently invested $28,600 of its $4.3 million in housing funding from the American Recovery and Reinvestment Act to apply reflective white paint on the roof of a public housing complex.
Simply put, a cool roof will save money for homeowners and businesses through reduced air conditioning costs. The real question is not whether we should move toward cool roof technology: it's why we haven't done it sooner.
Kudos to Dr. Chu for examining the science and embracing this sensible approach in combating climate change.
Dr. Art Rosenfeld is a member of the California Energy Commission.
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I live in Maine, in an 1870s-era three-story house with very steep slate roofs. How does this apply to my buildings?
I have an aluminum metal roof. It's considerably cooler than ashpalt shingles.
Or grow grass on the roof.
More than grass...grow an ecosystem up there.
At what latitudes is this method effective? What about winter?
In winter you want solar water heating to reduce your heating bill. Well, you really want that in summer, too. The net effect of not burning natural gas or using electricity to heat greatly outweigh the advantages of a light colored roof. Above mentioned calculations only compare one bad scenario (the way it is right now) with a slightly better one. They do not compare a slightly better one with the real solution. That's a very common device in American marketing to make people feel good about one thing hoping they won't think beyond the presented scenarios.
As far north as Chicago.... That is where I live.
The myth that cool roofs are detrimental is untrue. The data from Lawrence Berkley National Labs shows that.
Plus, in the case of my roof, there is no ice damming on a roof when the snow slides off within 72 hours of snowfall.
Your analysis is only partially correct. A properly designed roof is vented, so that there's an isolated air flow between the roof and the underlying layer of insulation. This air flow removes any heat retained by the roof and transfers it to the surrounding atmosphere (it also permits the dissipation of moisture that might have escaped from the living space into the insulation). This prevents the heat from the roof from penetrating through the insulation into the living space. Thus, the color of such a roof is almost meaningless to the heating and cooling of a properly designed building. However, your analysis is correct regarding heat retention that affects the surrounding environment. Also, most roofs aren't properly designed so the color of the roof would affect those buildings. Nonetheless, if we encourage people to build correctly engineered roofs then color will have a negligible effect for the individual homeowner.
My mother's home, built in the 1970s, with a black roof, small attic space and standard ceiling height and window size is completely unbearable in the summer without air conditioning. However, older homes, built before a/c with high ceilings, big windows and attics, and light roofs are a different story. How the home is sited N-S, and what kind of shade you have can help too.
Who not go a step further and actually replace all these roofs with solar panels? Why waste money on another layer of material that does nothing to generate energy? Solar panels can be coated with IR and UV reflective layers that get rid of all the radiation that the panel can not use to create electricity.
Excellent suggestion. Now let's get California to build pedestrian-friendly, mixed-use cities that cut both traffic congestion and vehicle miles travelled (VMT). The Southern California Association of Governments modeled congestion relief measures up to double-decking the freeways. Only mixed use (homes, commerce, offices, etc. in a single neighborhood) was effective. Widening roads simply fills up the extra lanes with "induced congestion" as people discover the additional road capacity.
This makes common sense. Who would make the inter-neighorhood commute if he could work, go to school, shop and live in the same neighborhood? The results of building such neighborhoods is that VMT drops between 1/3 and 2/3.
On the other hand, to do this, California would have to stop handing out the gigantic subsidies it gives land speculators. Imagine purchasing agricultural land, then getting it zoned for development. It's worth 100 times what you paid for it...at the cost of the relatively small planning fees. If you 1031 exchange out of this enormously profitable development, you don't even pay income tax on this outrageous profit.
The Germans require developers sell the ag land to the local government at the ag land price, then re-purchase it at the development land price. The result: German schools don't have to hold bake sales so they can have arts programs and after school sports. The arts budget for just the City of Berlin exceeds the U.S.' National Endowment for the Arts.
Let's stop developing ag land and put our cities on unproductive soils.
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